That's a really good question.
Choosing the visual design of a character is something that the character team does as a group. So in the Gorgon example, Oakley was very much part of the decision making. They take feedback from various people on the team - anyone who wants to give feedback. Decision makers are required to consider all feedback, but are not required to act on it. People are asked to give feedback honestly, without ego, while considering the high level goal of improving the overall product (Evolve) and making it as successful as possible.
Engineers might have technical concerns. Everyone could agree that a four legged monster concept looks awesome, for example, but four leg IK systems are more difficult to do than two. Choosing to go with four legs would impact the cost and possibly make us miss a milestone, or make people at the end of the pipeline have to crunch to meet the date. Animators might have feedback based on what they think they can animate well. Tentacles are difficult for them. Certain body types work better than others.
Sometimes people just think this one looks dumb, or that one looks cool, and that feedback is also considered.
Every department will be affected by a new character so everyone is expected to give feedback. Every department has a skill that we depend on, and nobody knows what each department needs better than the people in those departments.
So at the end of the day, a lot goes into the decision. It's not somebody at the top of the food chain making a decision that everyone else has to deal with. It's the team, trying to balance what is coolest and best for the product with what is actually feasible in the time we are given.